Mass spectrometer scanning

ABSTRACT

The positions of output peaks in a double focussing mass spectrometer due to metastable ions are separated by providing a high speed, limited duration, electrostatic scan when a peak is reached as a result of the magnetic scan. The electrostatic scan may be superimposed on the magnetic scan, or the magnetic scan may be interrupted for the duration of the electrostatic scan. The electrostatic scan is produced by varying the ratio between the ion accelerating voltage and the electrostatic deflecting voltage. Both the various voltages and the amplitudes of the output peaks may be digitized and recorded.

United States Patent Green et al.

[1 1 3,689,764 [451 Sept. 5,1972

[54] MASS SPECTROMETER SCANNING [73] Assignee: Associated Electrical Industries Limited, London, England [22] Filed: Oct. 9, 1970 [21] Appl. No.: 79,675

Related U.S. Application Data [63] Continuation of Ser. No. 686,390, Nov. 27,

1967, abandoned.

[30] Foreign Application Priority Data Dec. 1, 1966 Great Britain ..53,876/66 [52] U.S. Cl. ..250/41.9 ME, 250/419 G [51] Int. Cl. ..B0ld 59/44, H0 1 j 39/34 [58] Field of Search .....250/4l.9 D, 41.9 G, 41.9 TF, 250/419 ME [56] References Cited UNITED STATES PATENTS 2,945,126 7/ 1960 Brubaker et a1 ..250/41.9

. 3,416,073 12/1968 Gutow ..250/4l.9 3,010,060 11/1961 Lanneau ..250/41.9

FOREIGN PATENTS OR APPLICATIONS 957,084 5/ 1964 Great Britain ..250/41 .9

Primary Examiner-James W. Lawrence Assistant Examiner-C. E. Church AttorneyWatts, Hoffman, Fisher & Heinke [57] ABSTRACT The positions of output peaks in a double focussing mass spectrometer due to metastable ions are separated by providing a high speed, limited duration, electrostatic scan when a peak is reached as a result of the magnetic scan. The electrostatic scan may be superimposed on the magnetic scan, or the magnetic scan may be interrupted for the duration of the electrostatic scan. The electrostatic scan is produced by varying the ratio between the ion accelerating voltage and the electrostatic deflecting voltage. Both the various voltages and the amplitudes of the output peaks may be digitized and recorded.

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BRIAN M GREE BY MICHAEL EAEBEZ Ma i Mo ATTORNEYS.

PATENTEDSEP 51972 SHEET 7 0F 8 MSP cum uum INVENTORIE. BRIAN N. G/ZEEN BY MICHAEL 5/4/2552 ma/ M! M ATTOBNEYfi MASS SPECTROMETER SCANNING This is a continuation of application Ser. No. 686,390 filed Nov. 27, 1967 now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to mass spectrometers, and, more particularly, to a double-focussing mass spectrometer embodying a high speed, selectively operable, electrostatic scan for separating output peaks due to metastable ions.

In mass spectrometry, connectivity between output peaks in an ordinary low resolution mass spectrum can sometimes be inferred by observation of so-called metastable ions. These are due to ions of mass m decomposing during flight through the mass spectrometer to a daughter ion of mass m,, and giving rise to broad diffuse peaks, which occur usually at non-integral mass numbers. The positions m* of such peaks on the mass scale due to the process mf" "12 m is given by the relationship The observation of such peaks in a mass spectrum is of extreme importance in molecular structure elucidation, since they give valuable information as to the units which can be lost from a molecule in one step. For example, in the case where there are peaks corresponding to ions of mass numbers (Pl 5) and (P-43), which are respectively and 43 mass numbers lower than the molecular ion of mass number P, high resolution mass measurements have shown that these peaks are due to the loss of CR and C H O, respectively. The information now required is whether the (P43) ions have arisen (a) by a one-step process, e.g., the loss of C H O(in which case, the presence of an acetyl group (CI-I CO) may be inferred); or (b) by a two-step process, e.g., the loss of CH, and then by the loss of C0, in which case the structural interpretation is quite different. The two processes can be expressed as The presence of metastable peaks due to one or the other of these processes will give this information, since m* will be different for the two processes. For process For process (4):

different effects on the metastable spectrum depending on in which region in a mass spectrometer the decomposition occurs.

For purposes of explanation, consider a conventional Nier-Johnson mass spectrometer, although the explanation also applies to spectrometers such as the Mattuch-Herzog type. A Nier-Johnson spectrometer embodies an ion source and source slit, an electrostatic analyzer, an electromagnetic analyzer, and a collector slit and collector. These elements may be considered as defining five principal regions. The first region extends from the source slit to the electrostatic analyzer; the electrostatic analyzer itself defines the second region; the third region extends from the exit of the electrostatic analyzer to the entrance of the electromagnetic Decomposition in the third region gives rise to the normal metastable peaks at mass m*.

The fourth region provides a continuum stretching from mass m to mass m* due to ion decomposition in that region.

Ions undergoing decomposition in the fifth region in accord with equation (l) will be collected as m since no further deflection is involved.

If an ion of mass m, of kinetic energy eV (where e is the ion charge) decomposes in the first region to give an ion of mass m the latter ion willonlyhave m lm eV kinetic energy. This means that the electrostatic analyzer will discriminate against it in such a way that it will not only pass through a monitor slit assembly at the electrostatic analyzer exit; only those ions over a narrow band of energies centered on eV kinetic energy will do so. If, however, the accelerating voltage in the source is increased in the ratio m /m then ions of mass m will pass through the monitor slit assembly, but ions of mass m, will not. If the magnetic field of the magnetic analyzer has been adjusted so that, with an accelerating voltage V ions of mass m formed by transitions m m m in the source before acceleration pass through the collector slit, then ions of the same mass m formed by transitions occurring in the first region will again pass through the collector slit at an acceleration voltage V,, where V V m lm If, therefore, the magnetic analyzer is adjusted to tune ions of the mass m onto the final collector at an accelerating voltage V then by subsequently subjecting the accelerating voltage V to an independent scanning action (i.e., to a progressive variation unaccompanied by variations of the fields in the electrostatic and magnetic analyzers), output peaks will be obtained at the collector, which corresponds to ions of the mass m resulting from ion decomposition in the first region.

The same result can be obtained by keeping the acceleration voltage constant andscanning the deflection voltage in the electrostatic analyzer, although this introduces a change in the mass scale. In either case, the scanning has the effect of progressively varying the ratio between the deflection voltage and the acceleration voltage. The ratios between the normal acceleration (or electrostatic deflecting) voltage V and the voltages (V V at which peaks appear due to decomposition ions of the mass m can be accurately measured. The originating ion of the mass m for the process can then be uniquely determined by m V /V since m, is known.

It is obvious that the simplest system which can be used to perform this method is purely manual, i.e., the peak under investigation is tuned in, and the voltage is scanned manually. This is, however, very time-consuming (the order of one day is required to analyze a reasonably large molecule, where perhaps the mechanism for 100 values of m may be required). Accordingly, it is a general object of the present invention to provide apparatus for performing the described method of analysis in a matter of several hours or less instead of the many hours previously required by manual operation.

SUMMARY OF THE INVENTION In its simplest form, apparatus embodying the invention comprises means for applying a variable accelerating voltage (scanning voltage) to the ion source of a mass spectrometer after a collector output current peak has been found at a position corresponding to a decomposition ion having a mass m The accelerating voltage at the beginning and end of each scan is digitized and recorded. The values of the output current are sampled at regular intervals, digitized and recorded. Thus, the accelerating voltage at which each output current peak occurs can be computed, and the mass m, of the mother ion computed.

In another embodiment, the instantaneous accelerating voltage is measured and recorded each time an output current peak is detected and recorded. Alternatively, the accelerating voltage and spectrometer output currents are alternately sampled, digitized and recorded, whenever a predetermined collector output current level is exceeded.

In a further embodiment, all peaks in the mass spectrometer output greater in intensity than a predetermined value are analyzed. Normal scanning is accomplished by varying the field in the magnetic analyzer. As each peak is obtained, an accelerating voltage scan is made and the metastable peaks in the output are digitized and recorded along with the accelerating voltages. At the beginning of each voltage scan, the mass number derived as a function of the instantaneous magnetic field isalso digitized and recorded. From this information, the masses decomposing to each daughter mass can be found.

In still another embodiment, the magnetic field is held constant (magnetic scan stopped) during the accelerating voltage scan.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing a relatively simple embodiment of the invention;

FIG. 2 is a waveform diagram illustrating the operation of the embodiment of FIG. 1;

FIG. 3 is a schematic diagram of a voltage sweep generator, control unit and multiplexer used in the embodiment of FIG. 1;

FIGS. 4 and 5 are block diagrams of other embodiments of the invention;

FIG. 6 is a waveform diagram useful in understanding operation of the-embodim'entof FIG. 5;

FIG. 7 is a schematic diagram of a voltage sweep generator, control unit and threshold device used in embodiment of FIG. 5;

FIG. 8 is a block diagram of still another embodiment of the invention;

FIG. 9(a)-(c) are waveforms illustrating operation of the embodiment of FIG. 8;

FIG. 10 is a schematic diagram of various components used in the embodiment of FIG. 8; and

. FIG. 11 is a block diagram of a flux stabilizer that may be embodied in the system of the invention.

DESCRIPTION OF PREFERRED EMBODIMENT A block diagram of a relatively simple metastable ion scanning system for a mass spectrometer 10 is shown in FIG. 1. The spectrometer 10 may be of the Nier-Johnson type, such as the Model MS-9 available from Associated Electrical Industries Limited, London, Eng. Alternatively, it may be of the known Mattuch-Herzog type. The spectrometer includes a magnetic analyzer (not shown) to which magnetic deflection current is provided by a magnet current supply 1 1. A highly accurate linear voltage scan is selectively applied to an accelerating voltage supply 12 of the spectrometer 10 from a voltage sweep generator 14. The output voltage of the accelerating voltage supply 12 is provided to the spectrometer 10. A portion of that voltage is taken from a voltage divider comprising resistors 13a, 13b for recording. The varying output current from the mass spectrometer collector during the scan is recorded by a recorder 16 as a function of time and/or scanning voltage, being preferably digitized by an analog-to-digital (A/D) converter 18, and the digital information presented either on punched paper tape or magnetic tape or by teletype printer. In the embodiment shown in FIG. 1, spectrometer output is sampled under control of a multiplexer 20 at regular intervals during the scan, and the successive samples are digitized for recording. After manually tuning in an output peak due to a mass m at a fixed initial accelerating voltage V5 by adjusting the magnet current supply 11, the following operational sequence is initiated and is performed automatically under control of a suitable programmed control unit 22:

i. Offset the accelerating voltage V from Vh to (V AV) V'.

ii. Measure V' and record the value digitally.

iii. Initiate an accelerating voltage scan governed by the control unit 22.

iv. During the scan, pass the spectrometer collector output to the analog-to-digital (A/D) converter 18 at highly regular time intervals (e.g., every onetenth sec.) during the scan under control of the multiplexer 20, the operation of which is also governed by the control unit 22, the successive digital outputs being passed to the recorder 16 (e.g., printer, paper tape punch, or tape recorder).

v. At the end of the scan, measure the final voltage V" and record it as the last digital number.

vi. Reset the accelerating voltage to its original value An illustration-of the sampling of the analog output is shown in FIG. 2.

The accelerating voltage (V',,) corresponding to each collector current sample (L) can be calculated using V',, A V' X n, where n is the number of the sample counted from the beginning of the voltage scan and A V =(V"V') divided by the total number of samples taken.

The accelerating voltages (V V V at which peaks occur can be determined from the digitized information. One way is to establish the accelerating volt age at which the maximum collector current in each peak occurs. A more accurate value is given by the centroid of the peak calculated by V=( In V n/ In). The area of each peak can also be calculated. The centroid and area of each peak, as thus calculated, preferably by a computer, respectively represent the accelerating voltage and intensity of the peak. From these, the masses of the precursers can be readily calculated.

The foregoing method relies for its accuracy on the linearity of the voltage sweep,i.e., on the constancy of A V' over. the whole voltage scale. It has the disadvantage that a large number of output current values are taken per accelerating voltage scan, so that thresholding (i.e., elimination of outputs below a certain level or threshold) has to be done by the computer. It has the advantage, however, that both intensities and voltage ratios can be determined from a single run.

The accelerating voltage supply 12, which is used in all illustrated embodiments of the invention, is conventional. Basically, it comprises a rectifier and a high gain feedback amplifier, in which the output voltage follows a reference input voltage very closely. Preferably, the regulation of the accelerating voltage supply is at least 0.01 percent over a voltage range of 2-8 KV.

A suitable, but relatively simple, circuit of an accelerating voltage supply is described by Alfred O. Nier and shown in FIG. 6, page 403 of Review of Scientific Instruments, Volume 18, June, 1947. In the event that that particular voltage supply is used, the output of the voltage sweep generator 14 shown in the present application would be connected to the point labeled C in the reference figure in the Nier article.

Another suitable acceleration voltage supply is that which is sold as a part of the referenced MS-9-mass spectrometer by Associated Electrical Industries Limited.

In any event, the accelerating voltage supplied to the mass spectrometer 10 ranges from 2KV to 8 KV. The voltage divider comprising the resistors 13a, 13b provides a small percentage of that voltage to the multiplexer 20 for digitizing and recording when indicated.

The multiplexer 20, the AID converter 18 and the recorder 16 can take any of a number of forms. One simple and relatively inexpensive exemplary system is known as a data logger, a suitable one being available from Electronics Associates, Limited, Sussex, Eng. That system comprises an analog scanner (type MVPl00-0), which can serve as the multiplexer 20 shown in FIG. 1. The particular scanner referenced enables the output from up to 16 signal sources to be routed sequentially through it to the A/D converter. In the present application, only two input sources generally would be used. Those two sources are the divided-down accelerating voltage obtained from across the resistor 13b and the collector current output from the mass spectrometer. An additional input can be utilized as will be later described. In the relatively simple embodiment of the invention shown in FIG. 1, the multiplexer 20 as such may be eliminated and be replaced by a simple manually operated switch or relay, which routes one or the other of the two input signals to the A/D converter 18. This will be later described in connection with FIG. 3.

The A/D converter 18 may be a type number MDPl2l-0, which is also available as part of the data logger from Electronic Associates Limited. The recorder 16 may be of conventional type for providing an output in the form of punched paper tape, magnetic tape, etc.

FIG. 3 is a schematic diagram of certain components suitable for use in the embodiment shown in FIG. 1 and comprising the voltage sweep generator 14, the control unit 22, and the multiplexer 20. In the simplified embodiment of the invention shown in FIG. 1, the multiplexer and control unit may be combined in the manner shown in FIG. 3. The voltage sweep generator 14 comprises a conventional direct current (D.C.) operational amplifier 100 having a feedback capacitor 102 connected between its input and output. The output of the amplifier 100 is connected through a resistance chain to a source of +300 volts D.C. (not shown). The resistance chain comprises in sequence from the 300 volt supply to the amplifier a resistor 104, a potentiometer 106 and resistors 108, 110. One contact of a single-pole, double-throw switch 112 is connected to the juncture of the resistors 108, 110 and the other contact of the switch is connected trough a resistor 114 to the input of the amplifier 100. The pole of the switch 112 is connected to the output of the amplifier 100. Voltage output to be provided to the accelerating voltage supply 12 is taken from the movable arm of the potentiometer 106.

The combined control unit and multiplexer comprises the switch 112, a normally open switch 116 and a relay having an actuating coil 118. When the switch 116 is closed, it connects a source of 24 volts D.C.

" (not'shown) through the actuating coil 118 to ground.

The actuating coil 118 controls two single-pole, normally open relay sections 118a, 118b, and a single-pole, double-throw section 1186. When the coil 118 is energized and relay section 118a is closed, it connects the input of the amplifier through a resistor 120 to a movable arm of a potentiometer 122. The potentiometer 122 is connected in series with a fixed resistor 124 between the source of 24 volts an ground, and serves to adjust the slope of the voltage scan of the sweep generator 14. When the relay coil 118 is energized, the section 1l8b completes a circuit to energize the AID converter 18. When the coil 118 is de-energized, the section l18c serves to connect the divided-down output of the accelerating voltage supply 12 to the input to the A/D converter 18, and when the coil 118 is energized, the section 1180 connects the collector output of the mass spectrometer to the A/D converter.

The operational amplifier 100 may be of conventional type that is capable of producing 0-50 volts output. Many such devices are available on the open market, and it need not be further described. Values'of other components of the circuit shown in FIG. 3 are as follows:

I Capacitor The sequence of operations of the circuitry shown in FIG. 3 is controlled by the switches 112, 116. In operation, both switches are set to the positions shown and the potentiometer 106 is adjusted so that the ratio between the accelerating voltage and the electrostatic analyzer voltage of the mass spectrometer is normal. The ion having the mass m is then tuned onto the mass spectrometer collector by adjusting the magnet current supply 11 (FIG. 1). The switch 112 is then switched to its second position, which offsets the accelerating voltage; that is, switching the switch 112 from the position shown to its second position shorts out the resistor 110 and subtracts the voltage AV from the original voltage V (FIG. 2) determined by the adjustment of the potentiometer 106. This offset voltage may then be provided to the A/D converter 18 by momentarily closing a switch 126 connected in parallel with the relay section 1 18b. Thus that voltage is digitized and recorded by the recorder 16. The switch 116 is then switched to its closed position to energize the relay coil 1 18. When the relay coil 118 is energized, the relay section 118a closes, which initiates the accelerating voltage scan. Likewise, the section 1181) energizes the A/D converter, and section 118 c switches the input of the A/D converter 18 from the divided-down accelerating voltage to the collector current output of the mass spectrometer. When the accelerating voltage has risen to its desired value of approximately 8,000 volts, the switch 1 16 is opened, which freezes the accelerating voltage at its final value, de-energizes the A/D converter 18, and switches the input of the A/D converter 18 from the mass spectrometer output to the accelerating voltage. A single value of the final accelerating voltage may then be digitized by again momentarily closing the switch 126. After that, the switch 112 may again be switched to its first position to discharge the capacitor 102 through the resistor 114, and new mass m peak tuned in by manually adjusting the magnetic field. The sequence of operations may then be repeated.

The system shown in block form in FIG. 4 differs from that of FIG. 1 in that a peak top sensing device 24 is interposed between the mass spectrometer collector output and the digitizer constituted by the multiplexer 20 in conjunction with the analog-to-digital converter 18. The spectrometer collector output is supplied to a re-settable maximum-peak-height store 26 (which stores the maximum level reached by the collector since it was last reset), and also to the peak top sensing device 24. When the output has passed a maximum, the peak top sensing device 24 detects this and actuates the multiplexer 20 and the analog-to-digital converter 18. The instantaneous value of the accelerating voltage is digitized and recorded, followed by the maximum peak height store 26. After a suitable delay time in the control unit 22, and sufficient to allow the decreasing collector output to drop to zero, the maximum-peakheight store 26 is reset to zero so the store 26 is ready to store the next peak. This apparatus has an advantage in that only a few digital numbers are recorded, and

thresholding is automatically carried out. Also, the linearity of the voltage scan is relatively unimportant. It has the disadvantage that information on peak shape is not recorded, and the peaks must have relatively sharp tops for the peak top senser to operate satisfactorily. Because of statistical noise, the accuracy of measurement obtained is less than with methods and apparatus using centroids for the accelerating voltage determination.

The system shown'in block diagram in FIG. 5 is a further modification of thesystem of FIG. 1. In this case, the digitizer (multiplexer 20 and A/D converter 18) alternately samples the accelerating voltage and the mass spectrometer current output only after actuation by a threshold device 28. When the collector output of the mass spectrometer 10 rises above a predetermined value, the threshold device 28 detects this and, through the control unit22', activates the multiplexer 20, which, in this case, passes to the analog-to-digital converter 18 samples of the accelerating voltage (V) alternating with samples of the collector ion current (I), as shown in FIG. 6..The actual operating conditions are as outlined for FIG. 1, except for the threshold activation and the alternate multiplexing. The accuracy of this method depends on the linearity of the voltage sweep between consecutive recorded values of accelerating voltages. However, in order to obtain a reasonable number of digitized samples in the peak for determination of the voltage centroid, the digitizer must run faster than in either of the previous two systems. On the other hand, the advantages of a smaller number of samples and automatic thresholding still apply.

FIG. 7 is a schematic diagram of a combined control unit and voltage sweep generator for the embodiment of the invention shown in block form in FIG. 5. The voltage sweep generator 14 shown in FIG. 7 is identical to that shown andpreviously described in connection with FIG. 3. Therefore, identical components bear the same reference numerals and have the same values as previously set forth.

The apparatus shown schematically in FIG. 7 differs from that shown in FIG. 3 primarily in that it embodies a threshold device 28, and the control switches operate in a somewhat different manner.

As shown in FIG. 7, the threshold device 28 com prises a differential voltage comparator 200 that receives one input signal through a 47K resistor 202 from the collector of the mass spectrometer 10. The signal from the mass spectrometer 10 appears across a 2.7K resistor 204 connected between the first input of the comparator 200 and ground. A second input to the comparator 200 is provided from a movable arm of a 500 ohm potentiometer 206 connected in series with a 47K resistor 208 between a source of +24 volts D.C. (not shown) and ground. When the input signal provided from the mass spectrometer 10 to the comparator 200 exceeds the predetermined voltage provided from the movable arm of the potentiometer 206, a positive output signal is provided from the comparator. That output signal-is coupled through a 1K resistor 210 to the base of an NPN resistor 212. I

The emitter of the transistor 212 is grounded, and the collector of the transistor is connected through a relay actuating coil 214 to the source of +24 volts D.C.

The transistor 212 is non-conductive and no current flows through the relay actuating coil 214 until the output of the comparator 200 drives the base of the transistor positive with respect to ground. When that occurs, the transistor 212, acting as a switch, conducts and allows the coil 214 to be energized. When the relay actuating coil 214 is energized, normally open contacts of a relay section 214a close. When the relay contacts 214a close, it partially completes a circuit to energize the multiplexer 20 (FIG.

The comparator 220 may comprise a conventional Schmitt-tn'gger circuit, or an integrated circuit of the type recently developed. An example of the latter type is a differential comparator known as Model mA 710C, manufactured and sold by Fairchild Semiconductor Division of Fairchild Camera and Instrument Corp.,

vMountain View, Calif.

The control circuitry shown in FIG. 7 also differs from that shown in FIG. 3 in that only one switch is utilized rather'than two. As shown in FIG. 7, the control function is performed by a switch having two ganged sections 220a, 220b, each section having a single pole and three contacts. When the switch 220 is in its first position as shown, the resistor 114 is connected across the capacitor 102 in the sweep generator 14 by the first section 220a, and the second section 220b is open. In the second position of the switch 220, the section 220a disconnects the resistor 114 from across the capacitor 102, and shorts out the resistor 1 the second section 220b remains open. In its third position, the section 2204 still shorts out the resistor 110, and the second section 22% connects the actuating coil 222 of a relay between a 24 volt D.C. source (not shown) and ground. The coil 222 controls two normally-open single pole, single-throw-relay sections 222a, 222b. When the coil 222 is energized by the switch 220 being moved to its third position, the relay section 2220 connects the input of the amplifier 100 to the movable arm of the potentiometer 122 through, the resistor 120 as previ-- ously described with reference to FIG. 3. The relay section 222b closes to complete the circuit through the relay section 214a to enable the multiplexer 20.

In operation, with the switch 220 in its first position as shown, the potentiometer 106 is adjusted as previously described to set the accelerating voltage V and the mass m; is tuned onto the collector of the mass spectrometer by adjusting the magnet current of the magnetic analyzer. The switch 220 is then moved to its second position where the section 220a shorts out the resistor l 10 and reduces the accelerating voltage by approximately 5 percent. The switch 220 is then moved to its third position, in which the section 220a still shorts out the resistor 210. This, however, causes the relay energizing coil 222 to be energized through the section 220b, thus closing the relay contacts 222a, 222b. When the contact 222a closes, the voltage scan is started in the sweep generator 14. Closing the contact 222b completes the circuit to energize the multiplexer 20 when the relay section 214a closes.

When the collector current from the mass spectrometer l0 rises above a predetermined level, as determined by the setting of the arm of the potentiometer 206, the comparator 200 in the threshold device 28 provides a positive output signal to the base of the transistor 212. This causes the transistor 212 to conduct, thus energizing the relay coil 214. This in turn closes the relay contact 214a and enables the multiplexer 20. The spectrometer collector output current and the divided-down accelerating voltage are alternately digitized by the A/D converter 18 until the collector current of the spectrometer falls below the value set by the potentiometer 206. At that point, the end of a metastable peak has been reached. The switch 220 is returned to its first position, and the sequence of operations may be repeated for a new mass m peak.

In each of the previously described systems, the mass spectrometer operator has to select the daughter mass peaks (m manually before each voltage scan is made. This is time consuming and the operator is required to decide which peaks are to be measured. A further embodiment of the invention provides a fully automated system, in which all of the peaks in the mass spectrum greater in intensity than a predetermined value are analyzed. The basis of operation is as follows.

By variation of the magnetic field in the magnetic analyzer, a mass spectrum is scanned in the usual way. The preferred method of scanning is by decreasing the magnetic field exponentially. As each peak in the spectrum is detected in the collector output, an accelerating voltage scan is made and any resulting metastable peaks obtained in the output are digitized, together with the related accelerating voltage-magnitudes. At the beginning of each voltage scan, the mass number, derivable as a function of the instantaneous value of the magnetic field, is also digitized. The information can be recorded digitally, e.g., on magnetic or paper tape, for subsequent processing by a computer, or alternatively in this case as in the others, the output could be fed directly to a computer. The computer would determine the accelerating voltage at which each peak in the voltage scan occurred. From this information, the masses decomposing to each daughter mass can be found. By further processing, all the decomposition fragments could be arranged in an array to show the breakdown paths from the original molecule.

FIG. 9(a) shows how mass spectrometer collector ion current (represented by a curve 30) varies as the magnetic field is scanned over a small part of its normal maximum range in the absence of the accelerating voltage scan. In a complete mass spectrum, there may be up to 500-l,000 peaks, such as shown at 30', 30" which, for example, would be scanned in 40 minutes. For the system to be described to work successfully, it is necessary for each peak 30', 30", to have a flat top, i.e., for there to be a region where a change in magnetic field in the magnetic analyzer produces little or no change in collector ion current. Such a flat topped peak can be produced by suitable adjustment of the ion source and collector slits in the spectrometer. For example, on thementioned AEI MS-9 mass spectrometer, such a peak would be produced with a source slit width of 0.002 inch and a collector slit width of 0.008 inch. It is also' necessary to ensure that at the highest mass of interest the peaks 30', 30" are sufficiently separated so as not to interfere with one another. For example, at a resolution of 1,000, sufficient separation could be expected up to about mass 800 in the MS-9 spectrometer. In the preferred method of magnetic scanning, that is, exponential decay of the magnetic field, all the peaks have substantially the same shape and width in time.

A block diagram of one embodiment of an automated system is shown in FIG. 8, and will be explained in conjunction with the-waveforms shown in FIG. 9. Components shown in FIG. 8, which are common to those previously described, are identified by the same reference numerals and have the same values. As in the embodiments previously described, the magnetic analyzer of a mass spectrometer is supplied with magnet current from the magnet current supply 11. The ion source of the mass spectrometer is also supplied with accelerating voltage from the accelerating voltage supply 12, which selectively is caused to scan by the voltage sweep generator 14.

It is pointed out that the output of the mass spectrometer 10 is continuously supplied to the multiplexer 20, as is a signal from a mass marker 36. The mass marker 36 is connected to the magnet current supply 1 l, and continuously generates mass markers as a function of the magnetic field of the magnetic analyzer in the mass spectrometer 10. However, a mass marker is passed by the multiplexer 20 only when a signal is received from the first threshold detector 34 indicating that the threshold 32 has been passed and an accelerating voltage scan is starting. The mass marker 36 may be utilized with any of the embodiments shown in FIGS. 1, 4,5 and 8. Similarly, the collector output of the mass spectrometer 10 is passed by the multiplexer 20 only under command of a control signal from the control unit 38. It is further pointed out the control unit 38 comprises a plurality of switches and relays, which may be located in various components of the apparatus. They are grouped together in FIG. 8 as the control unit 38 for ease of explanation, and will be described in detail in connection with FIG. 10.

When the collector current output of the spectrometer 10 exceeds a predetermined threshold level 32 at a time t,, thefirst threshold device 28 is actuated. This causes a signal to be supplied through the control unit 38 to the voltage sweep generator 14 to decrease the accelerating voltage by a small amount, as shown by a decrease from a level 40 to a level 42 in FIG. 9(b). This corresponds to the change from Vfto (V A V) shown in FIG. 2. The signal from the threshold device 28 also energizes a first time delay device 44. The first time delay device 44 provides a time delay D,, as shown in FIG. 9, which is so adjusted that if the accelerating voltage remains constant the collector ion current 30 will reach a constant value on the peak top 30' before the end of the delay D at a time In the example, the time from t to t would be about 0.25 second.

At the time t, (i.e., at the end of the delay D the first time delay device 44 causes the control unit 38 to initiate an accelerating voltage sweep in the sweep generator 14 and also enables the multiplexer 20. During the period from t, to t the accelerating voltage scans upwardly, as shown by a curve 46, and peaks 47 produced by metastable transitions appear at the collector output of the mass spectrometer 10. When the peaks 47 are greater than a certain amplitude by adjustment of a second threshold device 48, signals are sent to the control unit 38 from the threshold device 48 to enable alternate values of collector current and accelerating voltage to be provided to the A/D converter 18 and thence in digital form to the recorder 16. The peaks 47 appearing in the mass spectrometer output are shown in FIG. 9(0).

A voltage sweep detector 50 also receives a signal proportional to the output of the voltage sweep generator l4, and, when the output voltage of the sweep generator 14 has reached a predetermined level, the voltage sweep detector'50 is actuated. When the voltage sweep detector 50 is actuated, two events take place. First, a signal is sent from the detector to a second time delay device 52 to initiate its operation. Second, a signal is sent to the control unit 38 The signal sent to the control unit 38 from the voltage sweep detector 50 causes themultiplexer 20 to be disabled, and resets the voltage sweep generator 14 to zero to remove the offset voltage from the accelerating voltage. Thus, the accelerating voltage decays, as shown at 54, to is original value V as shown at 40.

When, at a time t, after a delay D,, the time delay device 52 operates, the entire system is reset and the operating cycle is complete. At this time, the peak 30 which has just been voltage scanned will have passed off the collector slit of the spectrometer 10, but the next peak 30" will not have arrived at the collector. Thus, the system has gone through a complete cycle and is ready to accept the next peak which passes over the collector slit of the spectrometer as magnetic scanningproceeds in the normal manner.

The requirements for the collector amplifier bandwidth are different for the periods t t and 1 -1 t being the point in the previous cycle corresponding to t In order to achieve maximum signal-to-noise ratio in each of the two periods, it is desirable to have a much lower bandwidth for the period t -t the bandwidth required is about 10 cps and for the period 4, the bandwidth could be increased to 700 cps. In a refinement of the system, therefore, provision can be made for changing the bandwidth from the low to the high value at point t or in the region t -t and from the high to the low value at point t or in the region t t 'FIG. 10 is a schematic diagram of the control unit, threshold devices, voltage sweep generator and voltage sweep detector shown in block form in FIG. 8. The various individual units shown in FIG. 10 are enclosed by broken lines. However, because the control unit 38 shown in FIG. 8 comprises a plurality of relays and switches which are shown adjacent their related components, the control unit has not been indicated as a unit in FIG. 10. Also, because the various relays in the control unit have numerous sections located adjacent various components of the apparatus, it has not been possible to connect all of the sections comprising each relay without unduly complicating the drawing. Therefore, each relay actuating coil has been designated by a reference numeral, and various sections comprising the relay have been designated by the same reference numeral followed by a suffix a, b, c etc.

As shown in FIG. 10, a signal representing the collector current of the mass spectrometer 10 is provided to one end of a voltage divider comprising resistors 300,302 connected in series to ground. The juncture of the resistors 300,302 is connected as one input to the second threshold device 48 and through a switch 304 as one input to the first threshold device 28.

The switch 304 comprises two ganged single-pole, three-contact sections 304a, 304b, When the switch 304 is in its first or automatic position as shown, the pole of the section 304a is grounded, and the signal from the juncture of the resistors 300, 302 is connected through the section 304b to one input of the comparator 200 in the first threshold device 28. The second input to the comparator 200 is from the potentiometer 206 previously described in connection with FIG. 7, and sets the input signal level above which the threshold device provides an output signal. When the switch 304 is'in its second position, both sections are open, and, when it is in its third position, a signal from the juncture of the potentiometer 206 and the resistor 208 is provided as the first input to the comparator 200.

The output signal from the first threshold device 28 is supplied through a resistor 306 to the base of an NPN transistor 308. The emitter of the transistor 308 is grounded, and its collector is connected through a relay actuating coil 310 and a normally closed relay section 312 to a source of +24 volts (not shown). A normally-open relay section 310a is connected between the emitter and collector of the transistor 308 and acts as a holding contact for the relay actuating coil 310.

The output signal from the comparator 200 is also provided to the first time'delay device 44 and thence through a coupling capacitor 314 to the base of an NPN transistor 316. The emitter of the transistor 316 is grounded, and its base is also connected to ground through a resistor 318. The collector of the transistor 316 is connected to the +24 volt supply through a relay actuating coil 320 and a normally closed relay section 322a. A normally open relay section 320a is connected between the emitter and collector of the transistor 316 and serves as a holding contact for the relay actuating coil 320.

As previously mentioned, the collector output signal from the mass spectrometer is also provided from the juncture of the resistors 300, 302 to the second threshold device 48. The threshold device 48 comprises a comparator 324 similar to the comparator 200 embodied in the first threshold device 28. The signal from the juncture of the resistors 300, 302 is provided as one input to the comparator 324. Another input to the comparator 324 is from the movable arm of a potentiometer 326, connected in series with a fixed resistor 328 between the +24 volt supply and ground. The setting of the arm of the potentiometer 326 determines the level of the input signal from the mass spectrometer that is required to actuate the comparator 324. The output of the comparator 324 is provided through a resistor 330 to the base of an NPN transistor 332. The emitter of the transistor 332 is grounded, and its collector is connected through a relay actuating coil 334 to the +24 volt supply.

The voltage sweep generator 14 shown in FIG. 10 is identical to that previously shown and described in connection with P16. 3. However, its connections into the remainder of the circuit shown in FIG. 10 cause its operation to be somewhat different than that previously described. An input signal is provided to the operational amplifier 100 in the sweep generator 14 through a normally open relay section 320b and a series-connected resistor 336 from a movable arm of a potentiometer 338. The potentiometer 338 is connected in series with a resistor 340 between a source of 24 volts (not shown) and ground. As in the embodiments previously described, one end ofthe resistor 114 is connected to the input of the amplifier 100. However, in the present case, the other end of the resistor 114 is connected to one contact of a single-pole, twocontact relay section 310b, to one contact of a singlepole, two-contact relay section 3220. A second contact of the relay section 3l0b is connected to the pole of the relay section 322c, and a second contact of the section 322c is connected to the juncture of the resistors 108, 1 10. The pole of the relay section 310b is connected directly to the output of the amplifier 100. When the relay coil 310 is de-energized, the section 3l0b causes the resistor 114 to be connected between the input and the output of the amplifier (across the capacitor 102), and the capacitor 102 is discharged. When the relay coil 310 is energized, the resistor 114 is removed from the circuit and the resistor is shorted out (as suming that the pole of the-relay section 322c remains in the position shown).

The output of the sweep generator 14 is also connected to ground through series-connected resistors 342, 344. The juncture of the resistors 342, 344 is connected to the input of the voltage sweep detector 50 to provide a signal that is proportional to the voltage sweep provided by the sweep generator 14. The voltage sweep detector 50 comprises a comparator 346, similar to the comparators 200,324 previously described. One input to the comparator 346 is from the juncture of the resistors 342, 344, and the other input to the comparator is from a movable arm of a potentiometer 348. The potentiometer 348 is connected in series with a resistor 350 between the +24 volt supply and ground. Thus, the setting of the arm of the potentiometer 348 determines the bias level provided to the comparator 346 and thus determines the level of input signal required from the sweep generator 14 to actuate the voltage sweep detector 50 to provide an output signal.

The output of the voltage sweep detector is connected through a resistor 352 to the base'of an NPN transistor 354. The emitter of the transistor 354 is grounded, and its collector is connected through a relay actuating coil 322 and a normally closed relay section 312b to the +24 volt supply. A normally open relay section 322b is connected between the emitter and collector of the transistor 354'and acts as a holding contact for the relay coil 322.

The output of the voltage sweep detector 50 is also connected to the input of the second time delay device 52. The output of the time delay device 52' is connected through a coupling capacitor 356 to the base of an NPN transistor 358. The base of the transistor 358 is grounded through a resistor 360. The emitter of the transistor 358 is directly grounded, and the collector of the transistor is connected through a relay actuating coil 312 to the +24 volt supply.

The NPN transistors 308, 316, 332, 354, 358 act merely as switches. They are all normally in a non-conducting condition, and conduct only when a positive signal is applied to their respective bases from the first threshold device 28, the first time delay device 44, the second threshold device 48, the voltage sweep detector 50, and the second time delay device 52.

In considering the operation of the apparatus shown in FIG. 10, assume that the switch 304 is in its first or automatic position as shown. With the switch 304 in that position, a signal proportional to the collector current of the mass spectrometer 10 is provided through the switch section 304b to one input of the comparator 200 in the threshold device 28. When the amplitude of that signal exceeds the predetermined threshold 32 (FIG. 9), the comparator 200 is actuated to provide a positive output signal. That positive signal supplied to the base of the transistor 308 causes that transistor to conduct. Thus, a current path is established from the +24 volt supply through the normally closed relay contact 312a, the relay actuating coil 310 and the transistor 308 to ground. Energization of the relay actuating coil '310 causes the contact 310a to close, thus maintaining the coil 310 energized. When the coil 310 is energized, it also actuates the section 3l0b to short out the resistor 110 in the voltage sweep generator 14 and decrease the voltage provided from the movable arm of the potentiometer 106 to the accelerating voltage supply 12. As previously mentioned, this decrease amounts to approximately 5 percent of the accelerating voltage, as shown at 42 in FIG. 9(b).

The positive output signal from the first threshold device 28 also energizes the first time delay device 44. After a predetermined time delay t,, as determined by the time delay device44, several events occur. First, a positive signal is momentarily provided to the base of the transistor 316 through the capacitor 314 to cause that transistor to conduct. When the transistor 316 conducts, current flows through the normally closed relay section 322a, the relay actuating coil 320 and transistor to ground. Current flow through the relay actuating coil 320 closes the relay section 320a which maintains the relay coil 320 energized so long as the relay section 322a remains closed. In addition, the relay section 320b is caused to close, which energizes the voltage sweep generator 14. Simultaneously, the relay section 320c closes, which partially completes a circuit to enable the multiplexer 20. These events all occur at time t,, as shown in FIG. 9.

During the time period from t to t,, the accelerating voltage is increasing, as shown by the curve 46 in FIG. 9(b). If the spectrometer collector output current, which is provided as one input to the second threshold device 48, exceeds a predetermined level (determined by the setting of the arm of the potentiometer 326), the second threshold device 48 provides a positive output signal to the transistor 332. This positive output signal causes the transistor 332 to conduct, which causes current to flow from the +24 volt source through the relay actuating coil 334 and the transistor to ground. This closes the normally open section 3340, thus completely enabling the multiplexer 20, and allowing the output current values from the mass spectrometer 10 to be digitized and recorded. Alternate digitization of the values of collector current and accelerating voltage occurs and the values are recorded so long as the spectrometer output signals exceed the amplitude level set by the second threshold device 48 or until the voltage scan is terminated.

The voltage scan is terminated at time t when the signal provided to the voltage sweep detector 50 from the juncture of the resistors 342,344 exceeds the bias voltage provided from the arm of the potentiometer 348. When this occurs, the comparator 346 provides a positive output voltage, which is supplied to the base of the transistor 354 and to the second time delay device 52. The positive signal supplied to the base of the transistor 354 causes that transistor to conduct, thus energizing the relay actuating coil 322 through the normally closed relay section 312b. The relay section 322b closes, thus maintaining the coil 322 energized. When the relay coil 322 is energized, it has several effects. First the normally closed section 322a opens to deenergize the relay coil 320 and thus permit the sections 320a, 320b, 3200 to open. Opening the section 320b removes the input signal from the amplifier in the voltage sweep generator 14, and opening the section 3200 de-energizesthe multiplexer 20.'Simultaneously, the relay section 3221: changes its position to place the resistor 114 across the capacitorfll02 in the voltage sweep generator 14 and discharge the capacitor 102.

As previously noted, the output signal from the voltage sweep detector 50 is also provided to the second time delay device 52. At time after a predetermined time delay D the second time delay device 52 is actuated to provide a momentary positive signal to the base of the transistor 358. When this occurs, the transistor 358 momentarily conducts to energize the relay actuating coil 312 in its collector emitter circuit. When the relay coil 312 is energized, the section 312a opens, thus de-energizing the relay actuating coil 310. This permits the holding section 310a to open, and permits-the section 31% to return to the position shown in FIG. 10. Simultaneously, the relay section 312b opens to deenergize the relay actuating coil 322. This permits the section 322a to close again, permits the holding section 322b to open, and permits the section 3220 to return to the position shown. When this occurs, all relays are deenergized and all contacts again maintain the positions shown in the drawing. v v

When the switch304' is placed in its center or second position, the entire circuit shown in'FIG. 10 is in an off condition.

When the switch 304 is placed in its first or uppermost position, a single scan is initiated. This occurs because the voltage applied to the comparator 200 through the switch 304 will be slightly greater than that supplied from the potentiometer 208. Thus, the first threshold device 28 will provide a positive output which will initiate'a single scan. I

Values of components of the circuit shown in FIG. 10, and not previously set forth, are as follows:

One disadvantage of the foregoing system is that, because the magnet current continues to change during the time that the voltage scan is made, an error is introduced into the determination of the masses of the parent ions which break down to the daughter ions. A correction for the error could be applied in subsequent (computer) processing, but this could lead to undue complication and cost in the processing. This problem is avoided if the magnetic field scan is stopped during the accelerating voltage scan. The simplest method of stopping the magnetic field scan is to interrupt the progressive magnet current scan variation by which it is produced. In many magnet current supplies for mass spectrometers, the current flows through a resistor in series with the magnet energizing coil. The voltage developed across this resistor is compared with a reference voltage in a current regulator that acts to maintain a constant relationship between these two voltages. In order to vary the magnet current to effect a magnetic scan, the reference voltage is varied in a specified way according to the required scan law. For example, in order to produce an exponentially decaying field, the reference voltage may be derived from a capacitor and resistor in parallel. At the start of the scan, the capacitor is charged to the required voltage, and is then allowed to discharge through the resistor.- Such a method is well known, and to enable the current scan to be stopped it is onlynecessary to introduce appropriate switching to disconnectthe discharging resistor from across the capacitor. Other methods of scanning the magnet field can be arranged so that when the reference voltage is made constant, after previously scanning, the magnet current scan is stopped.

However, it is known that, if a variable reference voltage to a magnet current stabilizer is held constant after previously varying, the magnetic field continues to change for a time with a magnitude which depends on the scanning rate prior to making the reference voltage constant. This is due to the presence of eddy currents in the iron of the yoke of the magnet, and because the current in the highly inductive magnet circuit takes a finitetime to stabilize. On all but the very slowest scanning rates, the magnetic field changes, after stabilizing the reference voltage, by an amount which is excessive when applied to the present system.

In the present system, therefore, variation of the magnetic field is discontinued by means of a flux stabilizer, while the voltage scan' is carried out. The flux stabilizer is a well known device in other fields (e.g., in

analyzers of the nuclear magnetic resonance type). A suitable arrangement for the present purpose is shown in FIG. 11. A coil 60 is wound on a magnet 62 of the magnetic analyzer of the mass spectrometer, and arranged so that it intercepts a major part of the magnetic flux generated by main coils 64. Current through the main coils 64 is controlled by a conventional magnet current regulator 66. The coil 60 feeds into an integrator 68, the output of which is proportional to any magnetic flux changes that have taken place. That output is provided after any necessary amplification in an amplifier 70 to a second coil 72 wound on the magnet 62. The integrator 68 has a capacitor 74 connected between its input and output, and a normally closed switch 76 connected across the capacitor 74. The energizing current to the second coil 72 is arranged to produce a flux which counteracts any change in flux in the magnet.

The system incorporating this flux stabilizer operates as follows At time t,(FIG. 9), the flux stabilizer is made operational by opening the switch 76 to permit the capacitor 74 to charge. At this point, the field is held constant by the action of the coil- 72, and the voltage scan is carried out as previously described. The reference voltage input to the magnet current stabilizer feeding the scanning current to the main coils 64 may either be held constant during the voltage scan or may be allowed to continue scanning. The switch 76 could take the form of a normally closed relay section actuated by the coil 320.

At the end of the voltage scan (time t,,), the flux stabilizer would be made inoperative by reclosing the switch 76. After a suitable time delay (D,, FIG. 9), to enable the magnetic current stabilizer to restabilize by time the whole system would be ready to accept the next peak.

Although the invention has been described in its preferred forms with a certain degree of particularity, it is understood that the present disclosure of the preferred forms has been made only by way of example and that numerous changes in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and the scope of the invention.

We claim:

1. A scanning system for a double-focussing mass spectrometer, the spectrometer including ion ac- I celerating means having an accelerating voltage applied thereto, and electrostatic analyzer having a deflection voltage applied thereto, a magnetic analyzer having a variable magnetic deflection current applied thereto for producing a magnetic scan at a first scanning rate of an ion beam across a collector and producing ion current at said collector, the improvement in a scanning system comprising:

a. selectively operable sweep means for scanning the mass-charge ratio by scanning a ratio between said accelerating voltage and said deflection voltage at a second scanning rate substantially faster than said first scanning rate through a predetermined range of ratios, including a normal operating ratio for the mass spectrometer; and,

b. first switch means effective during at least a portionof said scanning variation for providing at least one first output signal representative of an instantaneous value of ion current at said collector.

2. The system of claim 1 wherein said sweep means varies said accelerating voltage to produce said scanning variation of said ratio.

3. The system of claim 2, further including second switch means for changing said accelerating voltage from a normal operating value to a second value prior to initiation of said scanning variation, said second value being offset from said normal value in a sense opposite to the sense of said scanning variation.

4. The system of claim 3, wherein said second switch means returns said accelerating voltage to said normal value upon termination of said scanning variation.

5. The system of claim 3, wherein said first switch means is effective at least at a beginning and ending of said scanning variation for providing second output signals indicative of instantaneous values of said ratios.

6. The system of claim 1, further including a flux stabilizer for producing a magnetic field in said magnetic analyzer for counteracting said magnetic scan during variation of said ratio between said accelerating and deflection voltages.

7. A scanning system for a double-focussing mass spectrometer, the spectrometer including ion accelerating means having an accelerating voltage applied thereto, an electrostatic analyzer having a deflection voltage applied thereto, a magnetic analyzer having a variable magnetic deflection current applied thereto for providing a magnetic scan of an ion beam across a collector, the improvement in a scanning system comprising:

a. selectively operable sweep means for scanning a portion of a mass spectrum by scanning a ratio between said accelerating voltage and said deflection voltage through a predetermined range of ratios,-including a normal operating ratio for the mass spectrometer; and,

b. first switch means effective during at least a first portion of said scanning variation for providing at least one first output signal representative of an instantaneous value of ion current at said collector and effective during at least a second portion of said scanning variation for providing at least one second output signal indicative of an instantaneous value of said ratio.

8. The system of claim 7, wherein said first switch means is effective during at least a portion of said scanning variation for alternately providing said first and second output signals.

9. The system of claim 8 wherein said sweep means varies said accelerating voltage to produce said scanning variation of said ratio.

10. The system of claim 9, further including second switch means for changing said accelerating voltage from a normal value to a second value prior to initiation of said scanning variation, said second value being offset from said normal value in a sense opposite to the sense of said scanning variation.

11. The system of claim 10, wherein said second switch means returns said accelerating voltage to said normal value upon termination of said scanning variation.

12. The system of claim 8, further including threshold means for enabling said first switch means when said ion current at said collector is greater than variation predetermined level.

13. The system of claim 12, further including a flux stabilizer for producing a magnetic field in said magnetic analyzer for counteracting said magnetic scan during variation of said ratio between said accelerating and deflection voltages.

14. The system of claim 7, further including a flux stabilizer for producing a magnetic field in said mag netic analyzer for counteracting said magnetic scan during variation of said ratio between said accelerating and deflection voltages.

15. The system of claim 7, further including a flux stabilizer for producing a magnetic field in said magnetic analyzer for counteracting said magnetic scan during variation of said ratio between said accelerating and deflection voltages.

16. A scanning system for a double-focussing mass spectrometer, the spectrometer including ion accelerating means having an accelerating voltage applied thereto, an electrostatic analyzer having a deflection voltage applied thereto, a magnetic analyzer having a variable magnetic deflection current applied thereto for producing a magnetic scan of an ion beam across a collector, the improvement in a scanning system comprising:

a. selectively operable sweep means for scanning the mass-charge ratio by varying a ratio between said accelerating voltage and said deflection voltage at a second scanning rate substantially faster than said first scanning rate through a predetermined range of ratios, including a normal operating ratio for the mass spectrometer; and,

b. first switch means effective during at least a portion of said scanning variation for providing at least one first output signal representative of an instantaneous value of ion current at said collector.

17. The system of claim 16, wherein said first switch means is also effective in response to said ion current substantially reaching said peak for providing a second output signal indicative of an instantaneous value of said ratio at that time.

18. The system of claim 17, further including a flux stabilizer for producing a magnetic field-in said magnetic analyzer for counteracting said magnetic scan during variation of said ratio between said accelerating and deflection voltages.

19. A scanning system for a double-focussing mass spectrometer, the spectrometer including ion accelerating means having an accelerating voltage applied thereto, an electrostatic analyzer having a deflection voltage applied thereto, a magnetic analyzer having a variable magnetic deflection current applied thereto for producing a magnetic scan of an ion beam across a collector, the scanning system comprising:

a. selectively operable sweep means for scanning the mass-charge ratio by varying a ratio between said accelerating voltage and said deflection voltage through a predetermined range of ratios, including a normal operating ratio for the mass spectrometer;

b. first switch means effective during at least a portion of said scanning variation for providing at least one first output signal representative of an instantaneous value of ion current at said collector; and,

. first threshold means for operating said sweep means to initiate said scanning variation after said ion current at said collector exceeds a first predetermined level as a result of said magnetic scan.

20. The system of claim 19 wherein said sweep means varies with accelerating voltage to produce said scanning variation of said ratio.

21. The system of claim 20, further including second switch means for changing said accelerating voltage from a normal operating value to a second value prior to initiation of said scanning variation, said second value being ofiset from said normal value in a sense opposite to the sense of said scanning variation.

22. The system of claim 21, wherein said second switch means returns said accelerating voltage to said normal value upon termination of said scanning variation.

23. The system of claim 21, further including first time delay means for initiating said scanning variation at a predetermined time after said accelerating voltage is changed to said second value.

24. The system of claim 23, wherein said second switch means returns said accelerating voltage to said normal value upon termination of said scanning variation.

25. The system of claim 23, further including second threshold means for enabling said first switch means during said scanning variation when said ion current at said collector is greater than a second predetermined level.

26. The system of claim 23, further including a flux stabilizer for producing a magnetic flux in said mag netic analyzer for counteracting said magnetic scan during variation of said ratio between said accelerating and deflection voltages.

27. The system of claim 19, wherein said first switch means is effective during at least a portion of said scanning variation for alternately providing said first and second output signals.

28. The system of claim 19, further including second threshold means for enabling said first switch means during said scanning variation when said ion current at said collector is greater than a second predetermined level.

29. The system of claim 19 further including a flux stabilizer for producing a magnetic field in said magnetic analyzer for counteracting said magnetic scan during variation of said ratio between said accelerating and deflection voltages.

30. The system of claim 19, wherein said first switch means is effective during at least a portion of said scanning variation for alternately providing said first and second output signals.

31. The system of claim 30, further including recording means for recording said first and second output signals.

32. The system of claim 31, further including means for digitizing said first and second output signals prior to recording. v

33. A scanning system for a double-focussing mass spectrometer, the spectrometer including ion accelerating means having an accelerating voltage applied thereto, an electrostatic analyzer having a deflection voltage applied thereto, a magnetic analyzer having a variable magnetic deflection current applied thereto for producing a magnetic scan of an ion beam across a collector, the scanning system comprising:

a. selectively operable sweep means for scanning the mass-charge ratio by scanning a ratio between said accelerating voltage and said deflection voltage through a predetermined range of ratios, including a normal operating ratio for the mass spectrometer; and,

b. first switch means effective during at least a first portion of said scanning variation for providing at least one first output signal representative of an instantaneous value of ion current at said collector and effective during at least a second portion of said scanning variation for providing at least one second output signal indicative of an instantaneous value of said ratio.

34. A scanning system for a double-focussing mass spectrometer, the spectrometer including ion accelerating means, an electrostatic analyzer and a magnetic analyzer positioned to deflect ions projected by said accelerating means at a first scanning rate, and collector means for receiving at least a portion of said projected ions and including monitoring circuit means for developing signals having values representative of the ions received, the improvement in a scanning system comprising:

first circuit means coupled to said accelerating means for applying an ion accelerating'signal to said accelerating means; I

- second circuit means coupled to said electrostatic analyzer for applying a deflection signal to said electrostatic analyzer; and,

control circuit means for scanning the mass-charge ratio by scanning the ratio between the value of said ion accele-rating signal and the value of said electrostatic deflection signal at a predetermined rate substantially different from said first scanning rate.

35. A scanning system as defined in claim 34 wherein said control circuit means includes sweep circuitmeans for substantially linearly varying said ratio between the value of said ion accelerating signal and the value of said electrostatic deflection signal.

36. A scanning system as defined in claim 34 wherein said control circuit means includes sweep circuit means for scanning said ratio between the value of said ion accelerating signal and the value of said electrostatic deflection signal between a predetermined upper level limit and a predetermined lower level limit.

37. A scanning system as defined in claim 34 wherein said control circuit means includes sweep circuit means for substantially linearly scanning said ratio between the value of said ion accelerating signal and the value of said electrostatic deflection signal between a predetermined upper level limit and a predetermined lower level limit. I

38. A scanning system as defined in claim 34 wherein said control circuit means includes sweep circuit means for scanning said ratio between the value of said ion ac-' celerating signal and the value of said electrostatic deflection signal by scanning the value of said electrostatic deflection signal.

39. A scanning system as defined in claim 34 wherein said control circuit means includes a voltage sweep generator means for developing a signal which varies in value at a preselected rate with respect to time; and,

third circuit means for applying a said varying signal to said first circuit means to cause the value of a said ion accelerating signal to vary in value at a preselected rate with respect to time to thereby vary said ratio between the value of said ion accelerating signal and the value of said electrostatic deflection signal at a preselected rate with respect to time.

40. A scanning system as defined in claim 39 including second monitoring circuit means for developing signals representative of the values of said ion accelerating signal; and,

fourth circuit means coupled to said first and second monitoring circuit means for providing an output indication representative of the value of both said ion accelerating signals and said signals developed by said first monitoring circuit means.

41. A scanning system as defined in claim 34 wherein said control circuit means includes a voltage sweep generator means for developing a signal which varies in value at a preselected rate with respect to time; and, 

1. A scanning system for a double-focussing mass spectrometer, the spectrometer including ion accelerating means having an accelerating voltage applied thereto, and electrostatic analyzer having a deflection voltage applied thereto, a magnetic analyzer having a variable magnetic deflection current applied thereto for producing a magnetic scan at a first scanning rate of an ion beam across a collector and producing ion current at said collector, the improvement in a scanning system comprising: a. selectively operable sweep means for scanning the mass-charge ratio by scanning a ratio between said accelerating voltage and said deflection voltage at a second scanning rate substantially faster than said first scanning rate through a predetermined range of ratios, including a normal operating ratio for the mass spectrometer; and, b. first switch means effective during at least a portion of said scanning variation for providing at least one first output signal representative of an instantaneous value of ion current at said collector.
 2. The system of claim 1 wherein said sweep means varies said accelerating voltage to produce said scanning variation of said ratio.
 3. The system of claim 2, further including second switch means for changing said accelerating voltage from a normal operating value to a second value prior to initiation of said scanning variation, said second value being offset from said normal value in a sense opposite to the sense of said scanning variation.
 4. The system of claim 3, wherein said second switch means returns said accelerating voltage to said normal value upon termination of said scanning variation.
 5. The system of claim 3, wherein said first switch means is effective at least at a beginning and ending of said scanning variation for providing second output signals indicative of instantaneous values of said ratios.
 6. The system of claim 1, further including a flux stabilizEr for producing a magnetic field in said magnetic analyzer for counteracting said magnetic scan during variation of said ratio between said accelerating and deflection voltages.
 7. A scanning system for a double-focussing mass spectrometer, the spectrometer including ion accelerating means having an accelerating voltage applied thereto, an electrostatic analyzer having a deflection voltage applied thereto, a magnetic analyzer having a variable magnetic deflection current applied thereto for providing a magnetic scan of an ion beam across a collector, the improvement in a scanning system comprising: a. selectively operable sweep means for scanning a portion of a mass spectrum by scanning a ratio between said accelerating voltage and said deflection voltage through a predetermined range of ratios, including a normal operating ratio for the mass spectrometer; and, b. first switch means effective during at least a first portion of said scanning variation for providing at least one first output signal representative of an instantaneous value of ion current at said collector and effective during at least a second portion of said scanning variation for providing at least one second output signal indicative of an instantaneous value of said ratio.
 8. The system of claim 7, wherein said first switch means is effective during at least a portion of said scanning variation for alternately providing said first and second output signals.
 9. The system of claim 8 wherein said sweep means varies said accelerating voltage to produce said scanning variation of said ratio.
 10. The system of claim 9, further including second switch means for changing said accelerating voltage from a normal value to a second value prior to initiation of said scanning variation, said second value being offset from said normal value in a sense opposite to the sense of said scanning variation.
 11. The system of claim 10, wherein said second switch means returns said accelerating voltage to said normal value upon termination of said scanning variation.
 12. The system of claim 8, further including threshold means for enabling said first switch means when said ion current at said collector is greater than variation predetermined level.
 13. The system of claim 12, further including a flux stabilizer for producing a magnetic field in said magnetic analyzer for counteracting said magnetic scan during variation of said ratio between said accelerating and deflection voltages.
 14. The system of claim 7, further including a flux stabilizer for producing a magnetic field in said magnetic analyzer for counteracting said magnetic scan during variation of said ratio between said accelerating and deflection voltages.
 15. The system of claim 7, further including a flux stabilizer for producing a magnetic field in said magnetic analyzer for counteracting said magnetic scan during variation of said ratio between said accelerating and deflection voltages.
 16. A scanning system for a double-focussing mass spectrometer, the spectrometer including ion accelerating means having an accelerating voltage applied thereto, an electrostatic analyzer having a deflection voltage applied thereto, a magnetic analyzer having a variable magnetic deflection current applied thereto for producing a magnetic scan of an ion beam across a collector, the improvement in a scanning system comprising: a. selectively operable sweep means for scanning the mass-charge ratio by varying a ratio between said accelerating voltage and said deflection voltage at a second scanning rate substantially faster than said first scanning rate through a predetermined range of ratios, including a normal operating ratio for the mass spectrometer; and, b. first switch means effective during at least a portion of said scanning variation for providing at least one first output signal representative of an instantaneous value of ion current at said collector.
 17. The system of claim 16, wherein said first switch means is also effective iN response to said ion current substantially reaching said peak for providing a second output signal indicative of an instantaneous value of said ratio at that time.
 18. The system of claim 17, further including a flux stabilizer for producing a magnetic field in said magnetic analyzer for counteracting said magnetic scan during variation of said ratio between said accelerating and deflection voltages.
 19. A scanning system for a double-focussing mass spectrometer, the spectrometer including ion accelerating means having an accelerating voltage applied thereto, an electrostatic analyzer having a deflection voltage applied thereto, a magnetic analyzer having a variable magnetic deflection current applied thereto for producing a magnetic scan of an ion beam across a collector, the scanning system comprising: a. selectively operable sweep means for scanning the mass-charge ratio by varying a ratio between said accelerating voltage and said deflection voltage through a predetermined range of ratios, including a normal operating ratio for the mass spectrometer; b. first switch means effective during at least a portion of said scanning variation for providing at least one first output signal representative of an instantaneous value of ion current at said collector; and, c. first threshold means for operating said sweep means to initiate said scanning variation after said ion current at said collector exceeds a first predetermined level as a result of said magnetic scan.
 20. The system of claim 19 wherein said sweep means varies with accelerating voltage to produce said scanning variation of said ratio.
 21. The system of claim 20, further including second switch means for changing said accelerating voltage from a normal operating value to a second value prior to initiation of said scanning variation, said second value being offset from said normal value in a sense opposite to the sense of said scanning variation.
 22. The system of claim 21, wherein said second switch means returns said accelerating voltage to said normal value upon termination of said scanning variation.
 23. The system of claim 21, further including first time delay means for initiating said scanning variation at a predetermined time after said accelerating voltage is changed to said second value.
 24. The system of claim 23, wherein said second switch means returns said accelerating voltage to said normal value upon termination of said scanning variation.
 25. The system of claim 23, further including second threshold means for enabling said first switch means during said scanning variation when said ion current at said collector is greater than a second predetermined level.
 26. The system of claim 23, further including a flux stabilizer for producing a magnetic flux in said magnetic analyzer for counteracting said magnetic scan during variation of said ratio between said accelerating and deflection voltages.
 27. The system of claim 19, wherein said first switch means is effective during at least a portion of said scanning variation for alternately providing said first and second output signals.
 28. The system of claim 19, further including second threshold means for enabling said first switch means during said scanning variation when said ion current at said collector is greater than a second predetermined level.
 29. The system of claim 19 further including a flux stabilizer for producing a magnetic field in said magnetic analyzer for counteracting said magnetic scan during variation of said ratio between said accelerating and deflection voltages.
 30. The system of claim 19, wherein said first switch means is effective during at least a portion of said scanning variation for alternately providing said first and second output signals.
 31. The system of claim 30, further including recording means for recording said first and second output signals.
 32. The system of claim 31, further including means for digitizing said first and second output siGnals prior to recording.
 33. A scanning system for a double-focussing mass spectrometer, the spectrometer including ion accelerating means having an accelerating voltage applied thereto, an electrostatic analyzer having a deflection voltage applied thereto, a magnetic analyzer having a variable magnetic deflection current applied thereto for producing a magnetic scan of an ion beam across a collector, the scanning system comprising: a. selectively operable sweep means for scanning the mass-charge ratio by scanning a ratio between said accelerating voltage and said deflection voltage through a predetermined range of ratios, including a normal operating ratio for the mass spectrometer; and, b. first switch means effective during at least a first portion of said scanning variation for providing at least one first output signal representative of an instantaneous value of ion current at said collector and effective during at least a second portion of said scanning variation for providing at least one second output signal indicative of an instantaneous value of said ratio.
 34. A scanning system for a double-focussing mass spectrometer, the spectrometer including ion accelerating means, an electrostatic analyzer and a magnetic analyzer positioned to deflect ions projected by said accelerating means at a first scanning rate, and collector means for receiving at least a portion of said projected ions and including monitoring circuit means for developing signals having values representative of the ions received, the improvement in a scanning system comprising: first circuit means coupled to said accelerating means for applying an ion accelerating signal to said accelerating means; second circuit means coupled to said electrostatic analyzer for applying a deflection signal to said electrostatic analyzer; and, control circuit means for scanning the mass-charge ratio by scanning the ratio between the value of said ion accele-rating signal and the value of said electrostatic deflection signal at a predetermined rate substantially different from said first scanning rate.
 35. A scanning system as defined in claim 34 wherein said control circuit means includes sweep circuit means for substantially linearly varying said ratio between the value of said ion accelerating signal and the value of said electrostatic deflection signal.
 36. A scanning system as defined in claim 34 wherein said control circuit means includes sweep circuit means for scanning said ratio between the value of said ion accelerating signal and the value of said electrostatic deflection signal between a predetermined upper level limit and a predetermined lower level limit.
 37. A scanning system as defined in claim 34 wherein said control circuit means includes sweep circuit means for substantially linearly scanning said ratio between the value of said ion accelerating signal and the value of said electrostatic deflection signal between a predetermined upper level limit and a predetermined lower level limit.
 38. A scanning system as defined in claim 34 wherein said control circuit means includes sweep circuit means for scanning said ratio between the value of said ion accelerating signal and the value of said electrostatic deflection signal by scanning the value of said electrostatic deflection signal.
 39. A scanning system as defined in claim 34 wherein said control circuit means includes a voltage sweep generator means for developing a signal which varies in value at a preselected rate with respect to time; and, third circuit means for applying a said varying signal to said first circuit means to cause the value of a said ion accelerating signal to vary in value at a preselected rate with respect to time to thereby vary said ratio between the value of said ion accelerating signal and the value of said electrostatic deflection signal at a preselected rate with respect to time.
 40. A scanning system as defined in claim 39 including second monitoring circuit means for devEloping signals representative of the values of said ion accelerating signal; and, fourth circuit means coupled to said first and second monitoring circuit means for providing an output indication representative of the value of both said ion accelerating signals and said signals developed by said first monitoring circuit means.
 41. A scanning system as defined in claim 34 wherein said control circuit means includes a voltage sweep generator means for developing a signal which varies in value at a preselected rate with respect to time; and, third circuit means for applying a said varying signal to said second circuit means to cause the value of a said electrostatic deflection signal to vary in value at a preselected rate with respect to time to thereby vary said ratio between the value of said ion accelerating signal and the value of said electrostatic deflection signal at a preselected rate with respect to time.
 42. A scanning system as defined in claim 41 including second monitoring circuit means for developing signals representative of the values of said electrostatic deflection signals; and, fourth circuit means coupled to said first and second monitoring circuit means for providing an output indication representative of the value of both said electrostatic deflection signals and said signals developed by said first monitoring circuit means.
 43. A scanning system as defined in claim 34 including a signal generating means coupled to said magnetic analyzer for applying a magnetic deflection signal to said magnetic analyzer; and, second control circuit means coupled to said signal generating means for scanning the value of said magnetic deflection signal at a first scanning rate.
 44. A scanning system as defined in claim 43 wherein said control circuit means includes variable circuit means for scanning the ratio between the value of said ion accelerating signal and the value of said electrostatic deflection signal at a scanning rate substantially faster than said first scanning rate.
 45. A scanning system as defined in claim 44 including first actuatable switching means coupled to said second control circuit means for terminating said scanning of said magnetic deflection signal when an output signal developed by said monitoring circuit attains a predetermined value; and, second actuatable means coupled to said first control circuit means for, upon termination of said magnetic deflection scanning, actuating said first control circuit means to initiate said ratio scanning.
 46. A scanning system as defined in claim 43 wherein said control circuit means includes sweep circuit means for substantially linearly varying said ratio between the value of said ion accelerating signal and the value of said electrostatic deflection signal.
 47. A scanning system as defined in claim 43 wherein said control circuit means includes sweep circuit means for varying said ratio between the value of said ion accelerating signal and the value of said electrostatic deflection signal between a predetermined upper level limit and a predetermined lower level limit.
 48. A scanning system as defined in claim 43 wherein said control circuit means includes sweep circuit means for substantially linearly scanning said ratio between the value of said ion accelerating signal and the value of said electrostatic deflection signal between a predetermined upper level limit and a predetermined lower level limit.
 49. A scanning system as defined in claim 43 wherein said control circuit means includes sweep circuit means for scanning said ratio between the value of said ion accelerating signal and the value of said electrostatic deflection signal by electronically scanning the value of said ion decelerating signal.
 50. A method of scanning with a mass spectrometer, the spectrometer including ion accelerating means, an electrostatic analyzer and a magnetic analyzer positioned to deflect ions projected by said accelerating means at a first scanning rate, and Collector means for receiving at least a portion of said projected ions and including monitoring circuit means for developing signals having values representative of the ions received, the steps comprising: applying an ion accelerating signal to said accelerating means; applying a deflection signal to said electrostatic analyzer; scanning the mass-charge ratio by scanning the ratio between the value of a said ion accelerating signal and the value of the deflection signal at a predetermined rate substantially different from said first scanning rate; and, providing output signals representative of the value of the signals developed by the collector means.
 51. A method of linearly scanning with a mass spectrometer, the spectrometer including ion accelerating means, an electrostatic analyzer and a magnetic analyzer positioned to deflect ions projected by said accelerating means at a first scanning rate, and collector means for receiving at least a portion of said projected ions and including monitoring circuit means for developing signals having values representative of the ions received, the steps comprising: applying an ion accelerating signal to said accelerating means; applying a deflecting signal to said electrostatic analyzer; linearly scanning the mass-charge ratio by scanning the ratio between the value of a said ion accelerating signal and the value of the deflection signal at a predetermined rate substantially different from said first scanning rate; and, providing output signals representative of the value of the signals developed by the collector means.
 52. A method of scanning with a mass spectrometer, the spectrometer including ion accelerating means, an electrostatic analyzer and a magnetic analyzer positioned to deflect ions projected by said accelerating means at a first scanning rate, and collector means for receiving at least a portion of said projected ions and including monitoring circuit means for developing signals having values representative of the ions received, the steps comprising: applying an ion accelerating signal to said accelerating means; applying a deflection signal to said electrostatic analyzer; scanning the mass-charge ratio by scanning the value of ion accelerating signals applied to the ion accelerating means at a predetermined rate substantially different from said first scanning rate; providing first output signals representative of the value of the signals developed by the collector means; and, developing second output signals having means representative of the values of said ions accelerating signal.
 53. The method as defined in claim 52 including the step of developing an output indication representative of the instantaneous values of both said first and second output signals.
 54. A method of scanning with a mass spectrometer, the spectrometer including ion accelerating means, an electrostatic analyzer and a magnetic analyzer positioned to deflect ions projected by said accelerating means at a first scanning rate, and collector means for receiving at least a portion of said projected ions and including monitoring circuit means for developing signals having values representative of the ions received, the steps comprising: applying an ion accelerating signal to said accelerating means; applying a deflection signal to said electrostatic analyzer; scanning the mass-charge ratio by scanning the value of said deflection signals applied to the electrostatic analyzer at a rate substantially different from said first scanning rate; providing fist output signals representative of the value of the signals developed by the collector means; and, developing second output signals having values representative of the values of said deflection signals.
 55. The method as defined in claim 54 including the step of developing an output indication representative of the instantaneous value of both said first and second output signals.
 56. A scanning system fOr a double-focussing mass spectrometer, the spectrometer including ion accelerating means, an electrostatic analyzer and a magnetic analyzer positioned to deflect ions projected by said accelerating means at a first scanning rate, and collector means for receiving at least a portion of said projected ions and including monitoring circuit means for developing signals having values representative of the ions received, the scanning system comprising: first circuit means coupled to said accelerating means for applying an ion accelerating signal to said accelerating means; second circuit means coupled to said electrostatic analyzer for applying a deflection signal to said electrostatic analyzer; control circuit means coupled to said first circuit means and having; a voltage sweep generator means for scanning the mass-charge ratio by generating a control signal having a value which varies as a preselected linear function of time between a predetermined upper and lower limit and at a rate substantially different from said first scanning rate; actuatable switching means for, upon actuation, actuating said sweep generator to initiate generator of a said control signal; variable means for varying the preselected rate at which a said control signal varies; third circuit means for applying a said linearly varying control signal to said first circuit means to cause a said ion accelerating signal to vary as a linear function of time; first monitoring circuit means for developing a plurality of first output signals each having a value representative of the instantaneous value of said ion accelerating signal; second monitoring circuit means for developing a plurality of second output signals each having a value representative of the instantaneous value of said collector monitored signals; indicator circuit means coupled to said first and second monitoring circuit means for developing an output indication representative of the values of both said first and said second output signals.
 57. A scanning system for a double-focusing mass spectrometer, the spectrometer including ion accelerating means, an electrostatic analyzer and a magnetic analyzer positioned to deflect ions projected by said accelerating means at a first scanning rate, and collector means for receiving at least a portion of said projected ions and including monitoring circuit means for developing signals having values representative of the ions received, the scanning system comprising: first circuit means coupled to said accelerating means for applying an ion accelerating signal to said accelerating means; second circuit means coupled to said electrostatic analyzer for applying a deflection circuit to said electrostatic analyzer; control circuit means coupled to said second circuit means and having; a voltage sweep generator means for scanning the mass-charge ratio by generating a control signal having a value which varies as a preselected linear function of time between a predetermined upper and lower limit and at a rate substantially different from said first scanning rate; actuatable switching means for, upon actuation, actuating said sweep generator to initiate generation of a said control signal; variable means for varying the preselected rate at which a said control signal varies; third circuit means for applying a said linearly varying control signal to said second circuit means to cause a said deflection signal to vary as a linear function of time; first monitoring circuit means for developing a plurality of first output signals having a value representative of the instantaneous value of said deflection signal; second monitoring circuit means for developing a plurality of second output signals each having a value representative of the instantaneous value of said collector monitored signals; indicator circuit means coupled to said first and second monitoring circuit means for developing an output indication representative of the values of both said first and said second output signals. 